Rick,
Envelopes load into ( adjustable ) paper trays, face down, seal to the right. They would print in landscape mode.
I think that could be done very easily. What sizes does your client currently use ? We could start with those, and then expand.
Since I work with the Printer class a lot, I believe I can actually create this quickly. I put it on my Agenda to do it Monday, and then after testing, I will post it here. Most envelopes are pretty standard, and you can easily print it on regular paper to test it.
Your client will need to have a printer that will feed envelopes. Otherwise they would have to manually hand feed them, and that would be very slow.
Personally, I use a Demo label printer, and it was easy to program that from FW. The only complication is finding the right way to print the address barcode. I researched it a few years back but didn't bother to implement it. That would make sending the envelopes faster. Otherwise the PO has to feed them through a machine that prints the barcodes IF it can read the address.
Tim